The present invention relates generally to multilayer papermakers' forming fabrics which can impart bulk to paper sheets formed thereon. It is particularly concerned with such forming fabrics which impart bulk due to the use of one or more weft yarns in the chosen paper side layer weave pattern which have a vertical dimension, measured in a direction substantially perpendicular to the paper side surface of the paper side layer, which is at least 1.25 times that of the corresponding vertical dimension of the remainder of the paper side layer weft yarns.
In the manufacture of paper products where a smooth sheet is required, such as for newsprint and other printing applications, the weave patterns selected for use in the paper side layer of the forming fabric upon which the sheet is formed are generally designed to maximize the planarity of the fabric and support the papermaking fibers evenly. For example, the objective of the composite forming fabrics disclosed in Seabrook et al. in U.S. Pat. No. 5,826,627 is to provide a generally planar surface upon which the paper sheet can be formed with minimal irregularities.
The forming fabrics disclosed by Seabrook et al. consist of a machine side (MS) layer of interlaced warp and weft yarns, and a separate paper side (PS) layer of interwoven warp and weft yarns. The PS weft consist of two sets of yarns: the first set, sometimes referred to as the primary weft, are weft yarns which interweave solely with the PS layer warp to form a portion of the papermaking surface. The second set, referred to as the “intrinsic weft binder yarns”, are pairs of yarns which, in each repeat of the fabric weave pattern interweave sequentially with the PS warp yarns to occupy an unbroken weft path in the PS, and interlace with at least one warp in the MS surface so as to bind the PS and MS layers of the fabric together. Each part of the unbroken weft path is separated from the adjacent parts of the unbroken weft path by at least one PS warp yarn. The weave pattern of the unbroken weft path may be the same as, or different from, that of the immediately adjacent primary weft yarns.
In those fabrics, the primary weft yarns of the first set are generally all circular in cross-section and have the same first diameter, and the intrinsic weft binder yarn pair members of the second set are all of the same second diameter, and this second diameter is generally less than or equal to the first diameter. The relative differences in diameter are limited by the need to avoid irregularities which would cause undesirable marking of the incipient paper web. Thus, the first and second yarn diameters should generally be relatively similar one to the other. For example, if the primary PS weft is about 0.13 mm in diameter, the intrinsic weft binder yarns will be from about 0.11 mm to about 0.13 mm in diameter.
Weave patterns for forming fabrics are also known in which variations of yarn sizes are used for different intended purposes. For example, U.S. Pat. No. 3,851,681 to Egan, and U.S. Pat. No. 5,181,117 to Huhtiniemi disclose forming fabrics using yarns of alternating diameter to provide improved wear resistance on the machine side surface. U.S. Pat. No. 5,074,339 to Vohringer discloses a double layer forming fabric woven in a 6-shed pattern with alternating large and small diameter weft yarns in the PS, grouped together in pairs to form a reverse funnel effect for improved drainage and sheet formation.
In the manufacture of cellulosic products such as towel and tissue, it is desirable to impart a measure of bulk to the sheet, most preferably with localized areas of bulk or non-uniformity. Various methods are known by which bulk can be imparted to localized areas by patterning of the forming fabrics. These methods include (1) adjusting the fabric weave pattern to create areas of high and low fabric yarn density and/or elevation, (2) applying a plastic resinous material onto the surface of the fabric in a predetermined pattern and (3) applying shaped plastic items to the surface of the fabric to impart or mold a desired shape into the sheet, and so on.
U.S. Pat. No. 6,841,037 (Scherb et al), U.S. Pat. No. 6,821,391 (Hay et al.) and U.S. Pat. No. 6,708,732 (Hay et al.) all generally disclose methods of forming paper products such as towel and tissue using fabrics which have systematically distributed areas of high drainage. However, a disadvantage of these fabrics is that the areas of high drainage have a relatively more open mesh which will tend to trap a portion of the papermaking fibers in the fabric, making it necessary to provide a cleaning mechanism so as to remove these fibers and prevent localized plugging of the fabric (see e.g. U.S. Pat. No. 6,841,037 at Col. 7, lines 28-56; U.S. Pat. No. 6,821,391, at Col. 3, lines 6-25). It will be appreciated that fabrics which are woven according to the teachings of any of Scherb et al. or Hay et al. must of necessity have distributed areas of high and low air permeability due to the nature of the disclosed yarn arrangements. Further, the warp yarn arrangement in fabrics constructed according to the teachings of Hay et al. must form “ . . . relatively long machine side floats” in the machine direction of the fabric (see U.S. Pat. No. 6,708,732 at Col. 4, lines 32-43). In addition, a portion of the weft yarns in fabrics woven according to the teachings of U.S. Pat. No. 6,708,732 are arranged so as to form an acute angle with the machine direction of the fabric.
It should also be noted that U.S. Pat. No. 6,841,037 claims a machine for producing a tissue web by means of a forming fabric having at least two zones of different wire permeabilities formed by “weaving threads of at least one different diameter and different weaving patterns”. However, this patent is entirely silent on the types of weaving patterns which could be used, or the different yarn diameters which would be suitable, and does not teach any manner of selecting either suitable weave patterns or yarn sizes. Further, U.S. Pat. No. 6,841,037 teaches the use of differing weave patterns within the fabric so as to obtain the zones of differing air permeability which will be about 5 mm or less in size, and thus does not disclose a forming fabric having a generally uniform air permeability.
WO 2005/035867 to Lafond et al. discloses a multilayer papermaking fabric having topographical differences created by a plane difference between at least two top weft yarns due to a different diameter, size, or shape. However, the disclosure lacks any specifics with respect to weave constructions and yarn sizes, and in particular is entirely silent as to suitable or preferred yarn sizes or weave patterns, and there is no indication of what the absolute difference in size, or feasible ranges, of the larger yarns might be as compared with the smaller weft yarns. Further, there is no disclosure of a preferred PS and MS weave combination that may provide beneficial results, nor is there teaching of what is meant by “hard” and “soft” materials in terms of the combinations of materials recited. Still further, there is no indication that any fabric disclosed has been successfully made or tested, or that it could be successfully produced without further experimentation.
U.S. Pat. No. 6,896,009 to Ward discloses a triple layer forming fabric which uses machine direction (MD) binder yarn pairs, in weave patterns which can include a variation in the diameter of the PS cross-machine direction (CD) yarns. However, the purpose of such variation, and for which a variation in yarn modulus can be substituted, is stated as being to compensate for the reduction in crimp of the MD yarns which otherwise may occur at the exchange points where one member of the pair enters the PS surface and the second member of the pair leaves the PS surface. Thus, the diameter variations contemplated are in the transverse direction to the binder yarn pairs. The patent does not teach the use of yarn diameter variations for yarns in the same direction as binder yarn pairs; and does not teach the use of such variations to provide a bulk enhancement for the paper sheet. To the contrary, the patent suggests that the use of the diameter variations as taught may improve sheet uniformity, by avoiding the steep “diving angle” of the yarns which are exchanging positions, i.e. by providing a smoother fabric surface.
U.S. Pat. No. 5,456,293; U.S. Pat. No. 5,542,455 and U.S. Pat. No. 5,817,213, each to Ostermayer et al., show the use of alternating diameter weft yarns in single layer forming fabrics to create bulk in tissue sheets. However, each these references relates only to single layer fabrics, and does not disclose or suggest any manner of using different weft yarn sizes in multilayered forming fabrics, such as double layer or composite fabrics.
For certain grades of tissue, it is preferred to form the sheet (which has a very low basis weight, in the range of about 5 grams per square meter, or gsm) on a patterned forming fabric as this provides areas of both high and low fiber density and correspondingly of high and low basis weight. Areas of the fabric which are raised relative to the fabric plane will produce a “bump” of low basis weight tissue, while depressions may create a “dimple” of a higher basis weight.
However, formation of such patterned areas by applying and securely attaching either a second material onto selected areas on the surface of a woven substrate, or by interlacing additional yarns according to a desired pattern, is a time-consuming and costly endeavour requiring a high degree of skill and complex machinery.
Thus, it would be desirable if a more cost effective and simple means were provided to impart bulk into the tissue sheet being formed on a multilayered forming fabric.
We have now discovered that it is possible to impart a measure of bulk into cellulosic products such as towel and tissue by forming them on fabrics having weave patterns which are similar or closely related to the forming fabrics disclosed by Seabrook et al. U.S. Pat. No. 5,826,627 being comprised of at least two layers of weft yarns, but in which the paper side layer weft yarns include both bulk enhancing (henceforth referred to as “BE yarns”) which have a relatively greater vertical dimension than the vertical dimension of the remainder of the weft yarns in the paper side surface.
As used herein, and as discussed further below, the term “vertical dimension” refers to the measurement of a yarn in the direction which will be substantially perpendicular to the paper side surface of the fabric when woven.
Surprisingly, we have found that fabrics woven according to weave patterns similar to those described in Seabrook et al., in U.S. Pat. No. 5,826,627, which have previously been intended to provide a very smooth and uniform sheet, may be used to generate high bulk in paper webs such as are intended for tissue or towel, by incorporating at least one BE yarn in each repeat of the paper side layer weave pattern. Weave patterns similar to those described in Seabrook et al., and noted above, include designs wherein the PS layer is comprised of two sets of weft yarns, one of which interweaves with the PS warp yarns only, and the other of which also contributes to the PS layer weave pattern and also interlaces with the MS warp yarns so as to bind the two layers together. FIG. 4 of the Seabrook et al. patent illustrates a typical weave pattern and yarn arrangement for fabrics of this type; other arrangements are possible within the scope of this invention.
According to the present invention, it has been found that bulk enhancement of the paper web formed on fabrics of the aforementioned type may be achieved by periodically inserting at least one BE yarn into the fabric weave pattern repeat in one or more of the PS weft positions, such as one or more of the primary weft positions, or in either or both of the intrinsic binder weft positions, in composite forming fabrics generally constructed according to the yarn arrangement described in the Seabrook et al. patent. The insertion may be regular or irregular in occurrence depending on the desired characteristics of the finished product.
Further, we have also found that it is possible to insert these BE yarns into any fabric construction having two layers of weft yarns, without jeopardizing the stability required for the MS layer of the fabric, or any other required fabric properties, provided that the vertical dimension of the BE yarns in the PS layer is compatible with that of the weft yarns used to form the MS layer weave structure. By “compatible” it is meant that the vertical dimension of the BE yarn does not introduce any areas of non-uniformity into the fabric structure such as would render it unstable, generally non-planar or otherwise unsuitable for use in the intended application due to irregularities in its construction induced by use of oversized yarns in the fabric.
Generally, we have found that in fabrics constructed according to the teachings of the present invention, from about 10% to about as much as 50% of the PS weft yarns may be BE yarns, and these may be woven according to any suitable pattern in which they will form floats over at least one, and preferably between two and eleven, warp yarns in the PS layer before interlacing with another warp.
We have also found that further improvement in bulk enhancement can be achieved by the selection of weave patterns for the fabrics of the invention so as to provide for variations in the float lengths of the different groups of weft yarns in the paper side surface of the fabrics. Thus, it is advantageous for at least one group of weft yarns to have longer paper side floats than the floats of the other weft yarns. This group having the longer floats can comprise all or some of one or all the sets of the BE weft yarns, or all or some of one or all the sets of the regular weft yarns.
Optionally, there may be more than one set of these BE yarns utilized in the construction of the PS surface of a fabric according to this invention. In other words, in the fabrics of this invention, it is possible to provide an array of BE yarns having a first set whose vertical dimension is e.g. 3 times the vertical dimension of the regular weft yarns, and a second set of intermediate BE yarns whose vertical dimension is about e.g. 1.5 times that of the regular weft yarns. As used herein, the phrase “regular weft” refers to those weft yarns which have dimensions in the range which would normally be used in the selected weave pattern for the PS surface of the fabric, have a smaller vertical dimension in the woven fabric than the BE yarns, and which have not been inserted into the fabric weave pattern so as to increase sheet bulk.
In a first broad embodiment of the invention, a double layer forming fabric is provided having two separate layers of weft yarns, located on each of the PS and MS layers of the fabric, wherein in the paper side layer the weft yarns comprise regular weft yarns and an array of BE yarns, which comprise between 10% to about 50% by number of the PS weft yarns, and each having a vertical dimension, measured in a direction substantially perpendicular to the PS surface of the woven fabric, from about 1.25 to about 3.0 times that of the regular PS weft yarns. The fabric may be of any weave construction as would be appropriate for the intended end use, but the PS weave pattern of the fabric is selected such that the float length of the BE yarns is at least one, and preferably between two and eleven, warp yarns.
Optionally, the array of BE weft yarns can comprise two sets, the first having the largest vertical dimension, and the second set having a vertical dimension which is less than that of the first set, yet nevertheless exceed that of the regular PS weft yarns by at least 1.25:1.
In a second more specific embodiment of this invention, the bulk enhancing forming fabric is a composite structure woven according to the general construction shown in U.S. Pat. No. 5,826,627 to Seabrook et al. In one version of this embodiment, the repeating weave pattern for the PS layer provides for the location in the PS of the BE weft yarns to vary between the positions of a or each primary weft, the first intrinsic weft pair member and the second intrinsic weft yarn pair member, in each successive repeat of the PS weave pattern. In this embodiment, from about 10% to about 50% of the paper side surface weft yarns are BE yarns whose vertical dimension is from about 1.25 to about 3.0 times that of at least one immediately adjacent weft yarn in the paper side surface of the fabric. Further, the PS weave pattern of the fabric is selected such that the float length of the BE yarns is at least one, preferably between two and eleven, warp yarns.
In further versions of the second embodiment, the repeating weave pattern for the PS layer provides for the BE yarns to be located in a single position, i.e. they are always primary weft yarns, or always the same ones of pairs of intrinsic weft binder yarns. In each of these versions of the second embodiment, the BE yarns have a vertical dimension that is from about 1.25 to about 3.0 times that of the other regular weft yarns and are located in about 10% to 50% of the PS weft yarn positions.